Devices of this class are known, in which a gas stream carrying particles, i.e., a flowing aerosol, is introduced into an evaporation unit, in which a working liquid, for example, butyl alcohol, is heated and evaporated. The evaporated molecules of the working liquid are deposited on particles to be measured, as a result of which the effective diameter of the particles to be measured increases. The gas stream is transported in this form farther in the direction of the outlet to the measuring unit, which detects the enlarged particles, for example, by optical devices.
The gas stream carrying particles is cooled with the molecules of the working liquid in a condensation unit. The particles of the gas stream now act as condensation nuclei, as a result of which their effective diameter is increased once more and the detection of the particles is simplified hereby. A device of this class with an advantageous evaporation unit is known especially from DE 10 2005 001 992 A1/EP 1 681 549 A2, whose disclosure content is made the subject of the present disclosure (related U.S. Pat. Nos. 7,543,803; 7,828,273; and 7,988,135 are hereby incorporated by reference).
It is common to all embodiments that the gas stream carrying particles, which is sent through the device, is always present as a laminar flow. The gas stream flows, when viewed in the cross section, in imaginary layers, there is no mixing of the layers with one another, so that no turbulent flows develop. The heating elements are provided in all embodiments of this class on the radial outer side of the channel, through which the gas stream flows, so that the highest percentage of evaporated working liquid is always present there and the concentration decreases rapidly inwards in the radial direction.
Directions will always be described below by a cylindrical coordinate system. The axial direction corresponds here to the longitudinal axis of the device from the aerosol inlet to the aerosol outlet. The aerosol device is usually, but not necessarily, oriented at right angles, the outlet being located above the inlet. A radial direction is at right angles to the axial direction, and so is a circumferential direction, which is also at right angles to the radial direction.
The response rate of the particle counter depends especially on the time that is needed for mixing the gas stream with the evaporated working liquid. Based on the laminar flow of the gas stream, an upper limit is set, on the one hand, to the flow velocity of the gas. On the other hand, a laminar flow has a greatly varying velocity profile in the radical cross section, and the flow velocity is highest in the radial cross section in the radial center of the flow cross section and drops towards the radial outer side. Thus, there is low flow velocity precisely in the area with the highest concentration of evaporated working liquid, which leads to reduced mixing. This considerably compromises the response rate of the particle counter.